1. The Field of the Invention
The present invention relates to a method and apparatus for monitoring the concentration of solids and fluorescent materials in waste water. In particular, the present invention involves the combined use of surface fluorescence and surface turbidity measurements to determine the concentration of suspended/dispersed solids and dissolved and dispersed fluorescent materials in waste water.
2. The Prior Art
Waste water streams contain residual components whose discharge is controlled by government regulation. When waste waters are discharged into the environment, various government regulations require monitoring of the discharged water for contaminants, in particular for the solid and organic content. The results of such monitoring must be kept on file for government review. Depending of the type of waste water discharge, severe fines can be assessed when contaminate levels exceed maximum permissible limits. In an effort to comply with government regulations, industries involved with waste water discharge have been searching desperately for analytical instruments that can measure water quality in real time. Real time measurements could then be used to prevent illegal discharges by providing early warnings of excessive contaminant levels.
Fluorescence is indicative of various contaminants whose presence is not desirable in discharged water streams. Turbidity is a measure of suspended particles in a water stream (whether due to organic or inorganic materials) and is a measure of water quality. The present invention involves using a single instrument to simultaneously measure the fluorescence and turbidity of a water stream to determine the water quality of that stream. Although this invention would be very useful in monitoring water discharge streams from petroleum production, it would also be useful to monitor any sort of water stream. A variety of instruments and methods have been tested for their ability to continuously monitor solids and fluorescent materials in various waste water streams. The following discussion emphasizes the prior art in the area of monitoring residual oil in water streams resulting from petroleum production, but the present invention is not so limited.
Focused ultra-sonic beams have been used to determine the particle content and particle size distribution of suspended solids in water streams. This technique is incapable of detecting dissolved fluorescent materials and cannot distinguish between, for example, dispersed oil particles and other types of suspended solids of similar size.
Optical methods, based on turbidity (light scattering), absorption, and fluorescence, have also been applied. Turbidimetry can indicate the suspended particle content of a water stream, by sensing light that is scattered from the particles as, for example, described in U.S. Pat. No. 3,309,956. Like the ultra-sonic technique, this method can only give a measurement of total particle content and cannot distinguish between dispersed hydrocarbons and other types of suspended particles.
Absorption methods are based on the ability of aromatic hydrocarbons to absorb ultraviolet (UV) light in a manner that is proportional to concentration. These absorption methods are primarily useful for determining ppm levels of dissolved aromatic hydrocarbons. Total dissolved hydrocarbons can be estimated if the aliphatic hydrocarbons, which do not absorb in the UV range, are in constant proportion to the UV absorbing aromatic hydrocarbons. Absorption techniques can also monitor dispersed hydrocarbons, but only if all the dispersed particles can be assumed to be 100% oil and if the particle size distribution is constant. Hybrid instruments are available which are capable of simultaneous absorption and turbidity measurements.
Fluorescence instruments also detect the aromatic components of petroleum hydrocarbons, but at much lower concentrations (parts-per-billion) than absorption instruments. Successful application of the fluorescence technique depends upon the same assumptions as discussed above for the absorption technique. The present invention combines the techniques of turbidimetry and fluorescence into one water monitoring instrument that is not subject to fouling of the optical surfaces by contact with the water stream.
All the optical measurement techniques require a means for light to come into contact with the water, and for transmitted, scattered, or fluorescent light to be detected. A variety of flow cells with optical windows or fiber optic probes are available for this purpose. The optical windows of flow cells and fiber optic probes are subject to fouling in most waste water streams because the suspended particles in the system (hydrocarbons, bacteria, etc.) cling to the optical surfaces and interfere with the transmission of light.
For fluorescence and turbidity measurements, non-contact sample cells are also available. Light is directed through open space to either a falling stream of water or a stabilized flowing water surface. Scattered or fluorescent light is then directed through an open space to a light detector. Fouling is eliminated because there is no direct contact between the water sample and the optical surfaces. One example of this optical arrangement is the Surface Scatter 6 Turbidimeter, described in the above mentioned U.S. Pat. No. 3,309,956, for the measurement of highly turbid samples.
An example of the falling stream system for the measurement of fluorescence is an instrument manufactured by Sigrist (Sigrist, Ennetburgen, Switzerland). An example of the falling stream system for the measurement of turbidity is also made by Sigrist.
In the prior art there are many examples of photometers, but only one is found that is capable of measuring absorbance, turbidity, nephelometric light, and fluorescence at the same time. There are no known examples of photometers which can make more than one type of measurement and are able to correct the fluorescence reading for error due to turbidity of the sample, or which use a sample cell that will not be easily fouled by a waste water stream.
U.S. Pat. No. 4,060,327 shows a device which is capable of measuring only absorbance and turbidity. It also uses a sample cell which would be susceptible to fouling in a waste water stream.
The system described in U.S. Pat. No. 4,426,154 is a device which measures fluorescent light, scattered light, and/or absorbed light, however it cannot measure nephelometric light.
Baker Instruments has a system which is capable of both absorbance and fluorescence measurements, but not nephelometric measurements.
U.S. Pat. No. 4,730,922 describes an instrument which is capable of measuring fluorescence, turbidity, absorbance, and nephelometry. However, a very crucial part of that instrument is the sample cell which would be very easily fouled by a waste water stream. Also, that invention does not seem to have the capability to correct the fluorescence measurement for the effects of turbidity.